Efforts have been made for a long time in underground mining to sense the boundary layer between the basement rock and the coal in running working operation, in order to adapt (cutting horizon setting) the underground winning system in as optimum a manner as possible, in accordance with the determined values for the boundary layer, to the profile of the coal layer above a rock layer which then forms the basement rock and is not to be extracted. In order to sense the boundary layer, optical detection sensors are usually used which have an optical sensor head and an optical window which is connected in front of the former, such as a sapphire window, in particular. By way of sensors in the sensor head, the different reflectance of basement rock and coal is utilized optically and fed as measured signal via optical waveguides to an optoelectrical converter and subsequently to an evaluation unit which calculates the profile of the boundary layer between basement rock and coal from the output signals.
In the detection systems for the profile of the basement rock/coal boundary layer which have previously usually been used in tests in underground mining, the optical detection sensor is installed into the mining winning system, with the result that there is permanently a measured signal for the current position of the mining winning machine by way of a detection sensor or optionally by way of two optical detection systems on the mining machine. A generic device of this type for a coal plough winning system is described in detail in DE 199 25 949 B1. The known detection sensor is arranged displaceably within a sensor carrier and is anchored as an exchangeable part in a recess in the plough body of the winning plough. The sensor head is biased in the direction of the basement rock by way of a compression spring, in order to ensure contact with the boundary layer which is to be sensed. Here, the sensor carrier is seated eccentrically with respect to the center axis of the plough body near one of the two pivotable tool carriers of the coal plough. The sensor head is equipped on the end side with a wearing shoe which is to protect the crystal window from destruction. However, it is shown during long-term operational use that the service life of the optical detection sensor is relatively low and the optical detection sensor has to be exchanged at the latest after two to three months. Experience during operational use shows that the service life decreases the stronger the pressing force of the sensor head by means of the compression springs onto the boundary layer which is to be sensed is selected to be, the more irregular the profile of the basement rock is on account of depressions or recesses, and the higher the plough speed is selected to be. Moreover, an eccentric arrangement of the optical detection sensor results in different loadings for the different planing directions.
In DE 44 14 578 C2, in order to determine the basement rock/coal boundary layer by means of optical detection sensors, the applicant has proposed not to transport the optical detection sensors with the winning system, but to integrate them into the front foot of the machine track of the machine guide, in order that the optical detection sensors are arranged permanently above the basement rock and can sense the boundary layer. The advantage of this substantially stationary arrangement of the optical detection sensors lies in considerably increased operational reliability on account of considerably lower loadings of the individual detection sensors. However, it is disadvantageous that only a small amount of installation space is available for the optical detection sensors on the machine track for the coal plough and the optical waveguides have to be laid in an unprotected manner at any rate partially on the underside of the machine track. Up to now, there has been no practical implementation of the arrangement of the detection sensors which is proposed in DE 44 14 578 C2.